MINOR: stktable: provide an unchecked version of stktable_data_ptr()

MINOR: stktable: provide an unchecked version of stktable_data_ptr()

stktable_data_ptr() currently performs null pointer checks but most
callers don't check the result since they know by construction that
it cannot be null. This causes valid warnings when building with
-Wextra which are worth addressing since it will result in better
code. Let's provide an unguarded version of this function for use
where the check is known to be useless and untested.